Geoscience Reference
In-Depth Information
5
Element transport
The theory of element transport is a way of representing the spatial changes in geochemical
properties in various contexts, such as movement in the ocean or mantle, the migration of
geological fluids or magmatic liquids within a rock matrix, or the attainment of chemical
and isotopic equilibrium among minerals within the same rock, etc. It is, in fact, a set
of rather complex theories involving some heavy-going mathematics, which we can only
touch lightly upon in this topic.
The essential concepts forming the core of this theory are those of conservation, flux,
sources, and sinks. A conservative property is additive and can only be altered by addition
or subtraction at the system boundaries or by the presence of sources and sinks. Mass or
number of moles are conservative properties; concentration is not: if a mole of salt is added
to a solution already containing one mole, the resulting solution will contain two moles,
regardless of how the salt is added. In contrast, two solutions of one mole per liter combine
to form two liters of a solution at one mole per liter. A flux is a quantity of something (mass,
moles, energy, etc.) crossing a unit surface per unit time. The most familiar of these fluxes
is volume flow, which is quite simply the velocity
v
(in cubic meters per square meter per
second). Mass flux is the mass content of volume flow, i.e.
ρv
ρ
, where
is the density of the
C
i
, where
C
i
is its local concentration in kilograms
(or moles) per kilogram. If the flux of a compound or an element changes suddenly at one
point, then a source or sink of this compound or element is present: generally, a chemical
reaction or radioactive process is responsible for this.
Transport from one point to another is either by advection or by diffusion. Think of
fish in a river. The general motion of water ensures advective transport of the fish; the
movement of the fish relative to the water is diffusive transport. Generally, advection is
medium. The flux of an element
i
is
ρv
